1. Field of the Invention
This invention relates to a rechargeable zinc cell containing an improved electrolyte which inhibits shape change in the zinc electrode while maintaining high discharge rates.
2. Description of the Related Art
Rechargeable cells containing zinc electrodes such as, for example, nickel/zinc, silver/zinc, and zinc/air cells, are of significant interest due to the abundance and, therefore, low cost of zinc, as well as low equivalent weight, high coulombic efficiency, resistance to corrosion, reversible electrochemical behavior, and reduced environmental disposal problems (e.g., in comparison to lead or cadmium).
When combined with a nickel positive electrode (NiOOH/Ni(OH).sub.2), the zinc cell also exhibits a high cell voltage of about 1.65 volts, which may be compared to 1.5-1.9 for a silver-zinc cell and 1.2-1.3 volts for a zinc-air cell.
However, while the above characteristics make such rechargeable zinc cells excellent candidates for applications such as electric vehicles and other portable power applications, the zinc electrode is not without problems. When used, for example, with electrolytes such as KOH (or a combination of KOH and LiOH, with the LiOH added to improve the performance of the nickel-containing electrode), to provide high ionic conductivity in the cell and, therefore, a high discharge rate, the zinc electrode tends to change its shape on its electrode support, apparently due to the high solubility of intermediate products formed as the zinc electrode discharges.
When the zinc electrode discharges, metallic zinc oxidizes to form zinc oxide. However, intermediate products such as zinc hydroxides or zincates, e.g., (Zn(OH).sub.4.sup.-2), form during the oxidation reaction which are very soluble in the KOH electrolyte. Furthermore, while the ionic conductivity of the electrolyte increases with increases in the concentration of the KOH electrolyte, the solubility of such zinc products in KOH also increases with the concentration of KOH.
The result is that when the cell is recharged, the reduced zinc may not occupy the same space as it did before it went through the discharge cycle. In particular, as shown in prior art FIG. 1 the edges of the original electrode gradually lose zinc electrode material, i.e., the total active area of the zinc deposit tends to decrease, resulting in a gradual loss in capacity of the cell. In fact, it is not unusual for a such a cell, after about 100-150 charge and discharge cycles to operate at less than 80% of its initial capacity
A number of possible solutions have been proposed, including the use of calcium in the zinc electrode to form an insoluble calcium zinc hydroxide such as CaZn.sub.2 (OH).sub.6 rather than the more-soluble zincates.
It has also been previously proposed to add other salts to the KOH electrolyte which would permit the use of a lower concentration of KOH while still providing the desired high ionic conductivity, to thereby inhibit shape change in the zinc electrode.
The use of alkaline-fluoride (KOH/KF) and alkaline-borate (KOH/K.sub.3 BO.sub.3) electrolytes to reduce the solubility of the zinc species was reported by Nichols and two of the inventors in this application, in an article entitled "Zinc Electrode Cycle-Life Performance in Alkaline Electrolytes having Reduced Zinc Species Solubility", which was published in Chem. Eng. Commun., Volume 37, pp. 355-379 in 1985. Although improvement in shape change or zinc electrode area retained was reported with the use of an alkaline-fluoride electrolyte, it was noted by the authors that in cells using such electrolytes the cell capacity is more sensitive to zinc area loss than in cells using standard electrolyte.
Thornton U.S. Pat. No. 4,247,610 disclosed a zinc battery aqueous electrolyte which contains 18-30% KF and 2-15% KOH with a KF plus KOH total concentration of 20-45%.
It has also been proposed to use potassium carbonate (K.sub.2 CO.sub.3) as an additive in a KOH electrolyte to reduce the shape change of the zinc electrode. McLarnon and Cairns, two of the three inventors of this invention, reported on the use of potassium carbonate, as well as a number of other additives to KOH electrolytes, to reduce shape change in zinc electrodes, in an article entitled "The Secondary Alkaline Zinc Electrode", published in the Journal of the Electrochemical Society Volume 138, No. 2, at pages 645-664, in February, 1991.
Jost U.S. Pat. No. 3,485,673 described and claimed a battery system comprising a positive electrode consisting of electrochemically active nickel material, a negative electrode consisting of electrochemically active zinc material, and an electrolyte consisting of an aqueous solution of potassium hydroxide and potassium carbonate.
However, while the use of KOH/KF or KOH/K.sub.2 CO.sub.3 electrolytes in a cell containing a zinc electrode does reduce the shape change in the zinc electrode, the amount of shape change and resulting charge capacity loss in such cell after 300 charge/discharge cycles is still unacceptable. Prior art FIGS. 2 and 3 show significant reduction in zinc electrode area in cells respectively using KOH/KF or KOH/K.sub.2 CO.sub.3 electrolytes after, respectively, 383 and 373 charge/discharge cycles.
There is, therefore, still a need for providing a rechargeable zinc cell which would exhibit low shape change in the zinc electrode, i.e., the capability of retaining greater than 80% of the initial capacity of the cell after 350 or more cycles of use, while still exhibiting a high electrolyte conductivity, i.e., a high charge and discharge rate.